Boiler thermostat

[Dambo]

Right, this is a strange one so bear with me!

I went to a neighbour's house today as her boiler packed in over the weekend and the plumber who came out to fix it thought that there might also be an issue with the electrics, more specifically the thermostat.

When I got there I found that bizarrely there were 2 thermostats in operation! Although the neighbour believed that one of them was old and wasn't wired up i.e. wasn't being used, it actually was. Here's what I discovered:

The old thermostat turns the boiler on and off. Switching the timer off doesn't stop this.

The new thermostat only comes on when the old thermostat and the timer are on. If either or both of these are off it goes off too.

There's only one cable going into the back of each thermostat.

The plumber seems stumped with the boiler too though so the wiring in it may not be right.

Any ideas as to how the old thermostat can switch the whole boiler on and off yet the new thermostat only works when the old thermostat and timer are switched on? bad day explode

 

[Apache]

Any ideas as to how the old thermostat can switch the whole boiler on and off yet the new thermostat only works when the old thermostat and timer are switched on? bad day explode

As simple as the old stat is wired in series to the new stat so when the old stat is off there is no power to new stat?

:|

 

[Dambo]

As simple as the old stat is wired in series to the new stat so when the old stat is off there is no power to new stat? :|

That was my original thought but there's only one cable going into the old thermostat i.e. no feed from that to the new thermostat.

 

[sparkytim]

Must be interconnection beween the two cables at some point, as already posted sounds like they have ended up in series.

Is there a wiring centre/junction box of some sort near the boiler or in the cylinder cupboard?

 

[batty]

No chance one is a frost stat?

 

[sparkytim]

Frost stat normally wired in parrellel to over ride other controls when they are all off

 

[wozz]

if i have read this correctly the old stat seems to be the master one, i have wired a system in a similar way with a external stat to control the system, but it doesn't sound like that is needed in this case.

 

[NozSpark]

Sounds like someone has been messing around with things they don't understand...

Personally I'd disconnect everything in the wiring centre, bell it all out and start from scratch

 

[Dambo]

Must be interconnection beween the two cables at some point, as already posted sounds like they have ended up in series.Is there a wiring centre/junction box of some sort near the boiler or in the cylinder cupboard?

There's no sign of wiring centres or junction boxes but I didn't get under the floorboards as the homeowner wasn't in and had just asked me to have a look at the thermostat itself.

Surely they can be wired in series though as if the old one's off the new one doesn't work but if the new one's off the old one still works ?:|

 

[NozSpark]

What sort of boiler is it... combi or heat??

 

[Dambo]

if i have read this correctly the old stat seems to be the master one, i have wired a system in a similar way with a external stat to control the system, but it doesn't sound like that is needed in this case.

How exactly would it be wired, in this instance?

 

[Dambo]

Sounds like someone has been messing around with things they don't understand...Personally I'd disconnect everything in the wiring centre, bell it all out and start from scratch

That's the most likely outcome but I just wanted to understand exactly how it's wired at present.

 

[Dambo]

What sort of boiler is it... combi or heat??

I think it's a combi boiler.

 

[NozSpark]

If it's a combi then it should be easy...

Power in and room stat... modern ones usually have a frost stat built in!

 

[Dambo]

If it's a combi then it should be easy...Power in and room stat... modern ones usually have a frost stat built in!

How does that explain the wiring irregularities?

 

[NozSpark]

TBH I wouldn't worry about how it's currently wired...